As is known in the art, a thermoelectric material refers to a material capable of directly converting thermal energy into electrical energy and vice versa or capable of cooling a material when a current is flowing in the desired direction. Such materials include, for example, semiconductor materials. In a thermoelectric generator, for example, the Seebeck voltage generated under a temperature difference drives a current through a load circuit. Typical thermoelectric generators employ a radioisotope, a nuclear reactor or a hydrocarbon burner as the heat source. Such generators are custom made for space missions, for example. Some materials such as tellurides and selenides are used for power generation up to a temperature of about 600.degree. centigrade (C). Silicon germanium alloys provide better thermoelectric performance above 600.degree. C. and up to about 1000.degree. C. With presently available materials, conversion efficiencies in the five to ten percent range are typically expected.
It would, however, be desirable to provide such thermoelectric materials having higher conversion efficiencies. Such devices may then be effectively employed in apparatus such as automobiles to thus increase the fuel efficiency of the automobile.
Superlattice structures, in general, are known and typically comprise a composite made of alternating ultrathin layers of different materials. Typically, the superlattice has an energy band structure which is different from, but related to, the energy band structure of the component materials. By the appropriate choice of materials (and other factors discussed below), a superlattice having a desired energy band structure and other characteristics can be produced. Superlattices have many uses, including, but not limited to, use in the field of thermoelectric power generation.
The fabrication of a superlattice by molecular beam epitaxy (MBE), or other known epitaxial growth techniques, is generally known. However, the choice of materials and the relative amounts of the materials which make up the superlattice are predominant factors in determining the characteristics of the superlattice. For use as a thermoelectric material in power generation applications, it is desirable to choose the materials, and their relative amounts, so that the thermoelectric figure of merit, ZT, is maximized.
The dimensionless thermoelectric figure of merit (ZT) is a measure of the effectiveness of the material for both cooling and power conversion applications and is related to material properties by the following equation: EQU ZT=S.sup.2 .sigma.T/K,
where S, .sigma., K, and T are the Seebeck coefficient, electrical conductivity, thermal conductivity and temperature, respectively. The Seebeck coefficient (S) is a measure of how readily electrons (or holes) can convert thermal to electrical energy in a temperature gradient as the electrons move across a thermoelement. The highest useful Seebeck coefficients are found in semiconductor materials with a high density of states at the Fermi level. In theory, to maximize the thermoelectric figure of merit ZT, one would try to maximize the Seebeck coefficient S, electrical conductivity .sigma. and temperature T and minimize the thermal conductivity K. However, in practice, this is not so simple. For example, as a material is doped to increase its electrical conductivity (.sigma.), bandfilling tends to lower the Seebeck coefficient S and the electronic contribution, K.sub.e to the thermal conductivity K increases. At a given temperature, the thermoelectric figure of merit ZT for a given material is maximized at an optimum doping level. In most materials, the thermoelectric figure of merit ZT is maximized at doping levels of approximately 10.sup.19 cm.sup.-3. Since increasing (or decreasing) one parameter may adversely increase (or decrease) another parameter, it is generally difficult to achieve higher values for ZT. Currently, the best thermoelectric materials have a maximum ZT of approximately 1. The ZT values are below one at temperatures both below and above the temperature at which they achieve the maximum value. For example, thermoelectric materials included among the best thermoelectric materials have a ZT of approximately 1 at a temperature of about 300.degree. K. while the value of ZT falls off at temperatures below and above 300.degree. K.
The figure of merit ZT in conventional thermoelectric materials is also related to the thermoelectric materials factor (b*) which may be expressed as: EQU b*=.mu.m*.sup.3/2 /K.sub.L
in which:
.mu. is the carrier mobility; PA1 m* is the density of states effective mass; and PA1 K.sub.L is the lattice thermal conductivity.
The precise relationship between the thermoelectric materials factor b* and the thermoelectric figure of merit ZT is relatively complex.
A superlattice provides the opportunity to enhance ZT for a number of reasons. Under appropriate conditions, the Seebeck coefficient of a superlattice increases as the thickness of a period of a quasi-two-dimensional superlattice decreases. The carrier mobility is generally increased by means of modulation doping and .delta.-doping, and this effect has been previously demonstrated in Si/SiGe strained-layer superlattices. Furthermore, the lattice thermal conductivity of a small-period superlattice is expected to be substantially lower than the average of the component materials because of augmented phonon-interface scattering effects.
In view of the above, it would be desirable to provide a superlattice structure which has a thermoelectric figure of merit which increases with increasing temperature above the maximum thermoelectric figure of merit achievable for bulk SiGe alloys.